Yes. If newbies can send messages, there should be a lot of spam and scam attempts through private messages. Since it is easy to create new accounts, scam attempts possibly to be very massive. Disable private messages for newbies is a good idea, I also think it is better to limit the number of posts made by newbies daily as well. It will minimize the chance of massive spam and scam attempts. Although it seems to discriminate against newbies, it is a proper way to protect our forum.